Gulf spill: How 3.6 million barrels of crude can disappear

Three quarters of some 4.9 million barrels of crude disgorged into the Gulf of Mexico from BP's ruptured oil well has been neutralised by nature or human efforts, according to a US government report

Only 26 percent of the oil remains close to its original form, floating on the ocean water or suspended under the surface, said the report by the National Oceanic and Atmospheric Administration (NOAA).

The other 74 percent has either evaporated or been burned, skimmed, recovered from the underwater wellhead or dispersed, a panel of government and independent scientists found.

"Three quarters is not surprising, indeed it is very consistent with our own preliminary estimates," said George Peigne, an expert on ocean oil spills at CEDRE, a French government pollution research centre.

"Indeed, it is possible that more than 75 percent has been eliminated," he told AFP.

Here is how the NOAA estimates breaks down:

REMOVAL AT SOURCE: After several false starts, some 17 percent of the oil was recovered directly from the busted wellhead a mile below the ocean surface using a riser pipe insertion tube and so-called "top hat" systems.

BURNING AND SKIMMING: Up to five percent of the crude was burned as it floated on the water, and another three percent was skimmed off the surface by a small armada of boats and ships.

NATURAL DISPERSION: NOAA estimates that 16 percent of the crude was dispersed naturally into the ocean water. As the oil gushed out of the well at high pressure, some of it sprayed apart into droplets less than 100 microns across, the diameter of a human hair.